Put the skewers in a bowl of water to soak. Tip the couscous into a bowl. Pour in 150ml boiling water. Cover with a plate or cling film and set aside to soak.
While the couscous soaks, tip the walnuts into a dry frying pan. Set over a medium heat. Toast for 2 mins, shaking the pan every so often, till the nuts have browned. Tip them onto a chopping board and leave to cool for 1-2 mins.
Finely chop the walnuts and tip into a bowl. Finely chop the sorrel and mint leaves and add to the walnuts. Finely grate in the lemon zest. Squeeze in the juice from half the lemon. Halve the chilli. Scoop out the seeds and white bits. Finely chop it and add to the bowl. Add a pinch of salt and pepper to the bowl and stir everything to make a salsa. Set aside.
Heat your grill to high. Halve the pepper and scoop out the seeds and white bits. Chop into chunks around 2cm across. Pop the pepper chunks in a dish with the fish pie mix. Add 1 tsp oil and a pinch of salt and pepper. Turn to mix.
Thread the pepper and fish pieces onto the skewers, alternating them as you go. Lay the fish and pepper skewers on a rack in a grill pan or on a baking tray. Grill the fish and pepper skewers for 8-10 mins, turning once or twice, till the fish is pale pink or white and cooked through.
While the fish cooks, drain the couscous if there is any excess water. Toss the couscous with the watercress and some salt and pepper. Serve the skewers with the couscous and the sorrel pesto, with the remaining lemon half sliced into wedges.